Conductive polyvinylchloride (PVC)-based molding compositions suitable for compression molding of video discs have been disclosed by Fox et al in copending application Ser. No. 818,279, filed July 25, 1977, now abandoned. The molding composition contains sufficient amounts of conductive carbon black particles to impart a maximum resistivity of 500 ohm-cm at 900 MHz. Various molding compositions disclosed contain from about 15 to about 35 percent by weight of the molding composition of conductive carbon black. Because of the high solids content, the molding compositions were stiff and difficult to mold.
These highly conductive PVC molding compositions were modified by Khanna as disclosed in copending application, Ser. No. 818,716, also filed July 25, 1977, now U.S. Pat. No. 4,151,132. Large amounts of a variety of lubricants, processing aids and modifiers were added to a mixture of PVC resins to improve the processability. However, these compositions have less than the desired dimensional stability at elevated temperatures.
Martin et al, in copending application Ser. No. 3,363, filed Jan. 15, 1979, now U.S. Pat. No. 4,228,050, disclose further improvements to the molding compositions whereby video discs with improved dimensional stability at temperatures up to 130.degree. F. (54.4.degree. C.) and excellent processability can be molded. These compositions have a lesser total amount of additives than the compositions of Khanna, and also limit the amount of liquid additives that can be present. These molding compositions comprise a PVC based resin; conductive carbon black in amounts necessary to obtain the required resistivity; from about 1.5 to 4 percent by weight of stabilizers; from about 1-3 percent by weight of at least 2 lubricants; and up to 10 percent by weight of additional modifiers, with the proviso that not more than 5 percent by weight of liquid additives are present.
For molding compositions containing large quantities of additives, the materials must be blended in a high intensity mixer such as a Henschel, Welex or Papenmeier type mixer. This type mixer is bowl shaped and has a high speed agitator projecting from the bottom whereby the ingredients are pulled into a vortex in the center of the agitator and whirled around to the walls of the bowl. These mixers develop a great deal of shear, raising the temperature so that a uniform dispersion of the ingredients is achieved. The resin and solid additives are generally mixed to a temperature of at least about 120.degree. F. (48.9.degree. C.), liquid ingredients are added, and mixing continued to a temperature of 160.degree. F. (71.1.degree. C.) or higher. After cooling down to about 80.degree.-120.degree. F. (26.7.degree.-48.9.degree. C.), the blended mixture is plasticated in a Banbury type mixer or an extruder where the mixture is subjected to high temperatures of about 300.degree.-500.degree. F. (148.9.degree.-260.degree. C.) with high shear to form a dough-like mass which can be formed into tubing or solid strands which are cut into pellets for storage.
Excellent video discs can be made from the above compositions which are dimensionally and environmentally stable. No staining is apparent on the discs, replication of the minute information pattern is excellent and playback is of high quality.
However, difficulties were encountered when the above process was scaled up to commercial quantities. Because of the low bulk density of the preferred carbon black, it is very difficult to mix in with the PVC resin and some of it is lost. Also because of the low bulk density of the carbon black, it is difficult to weigh the carbon black accurately and batch to batch variations in the amount of carbon in the molding composition were noted. In addition, the dispersion of the carbon black varied from batch to batch. On investigation it was found that the preferred carbon black has a wide particle size distribution; although the innate particle size of the carbon black is very small, the carbon particles agglomerate into larger particles which do not break up in a high intensity mixer. This large particle size leads to non-uniformity in playback performance of the video discs, to defects in the disc surface and even to scratching of the stampers during molding.
Thus, a process to improve the uniformity of the above molding composition and to improve the dispersion of large amounts of carbon black particles in a PVC resin was sought.